3,175 research outputs found
On deformations of D-manifolds and CR D-manifolds
No full textIn this paper we study the problem of small deformations of D-structures on compact manifolds. We prove that the integrability condition, namely the classical Maurer-Cartan equation, can be viewed as a condition on the space of skew-symmetric derivations of the algebra ∧^{0,∗}_K(M). The last part of the paper is devoted to the study of small deformations of strictly CR D-manifolds, and we consider explicit examples of such CR D-manifolds
Optimization of the pressure drop produced during CO2 replacement in hydrate reservoirs: Balance between gas removal and preservation of structures
No full textThe replacement of methane molecules with a theoretical equal number of CO2 molecules, is one of the most
promising opportunities for the exploitation of natural gas hydrate reservoirs. In this work, the effect of
depressurization on CH4 hydrates was analysed in depth. Different pressure drops, from 5 to 15 bar were tested
and described in terms of free gas removed (desired event) and hydrates destroyed (undesired event) before the
injection of carbon dioxide. A few pronounced pressure drop might be not enough to ensure an effective
replacement, while an excessive one might be not sustainable due to the high quantity of hydrates destroyed
before the process. In this work, variation of hydrates and free gas was evaluated before and after depressur ization; it was then compared and employed to define an optimum range of pressure drop to optimize the
process. It was proved that the quantity of hydrates destroyed increases with the pressure drop imposed, while
the removal of free gas shows a maximum. When depressurization was close to 10 bar, only 18.57% of hydrates
dissociated, while the quantity of free gas removed was three times higher (59.27%). Finally, the CO2/CH4
replacement process was carried out with the optimal depressurization degree previously defined
Hydrate formation as a method for natural gas separation into single compounds: a brief analysis of the process potential
No full textIn both natural gas and petroleum reservoirs, the extracted gas is not only composed of methane: a variable and significant quantity of
other compounds, such as different hydrocarbons (ethane, butane, pentane, propane, etc.), inert gas (nitrogen), and toxic and corrosive
molecules (i.e., carbon dioxide and hydrogen sulfide), are present. In order to reach commercial specifications, natural gas has to be
treated, in particular for reaching the minimum gross calorific value required and decreasing CO2 and H2S presence under the
respective tolerance values. To do this, several different treatments are commonly applied, like inlet separation, sweetening, mercury
removal, dehydration, liquid recovery, and, finally, compression for its transportation. Considering the growing demand and the
necessity of exploiting also lower quality natural gas reservoirs, in the present paper, an original solution, for performing a gas
treatment, is proposed and analyzed. It consists of promoting hydrates formation for both different compounds separation and gas
storage. The greatest part of chemicals commonly present in natural gas is capable to form hydrates, but at different thermodynamic
conditions than others. Parameters such as the typology of stored compound and the formation process efficiency are mainly related to
partial pressure of each element. Here, the present strategy has been explored and the results achievable were shown. In particular,
different possible natural gas compositions were taken into account and specifications required for gas commercialization were
considered target of the process. Results led to different possibilities of raw gas treatment: in some cases, gas separation led to
contemporary CH4 storage into hydrate structures, while, in the presence of different mixture compositions, contaminants were
trapped into water cages and methane (and, eventually, other hydrocarbon compounds) remained in the gas phas
Experimental investigation on the possibility of defining the feasibility of CO2/CH4 exchange into a natural gas hydrate marine reservoir via fast analysis of sediment properties
No full textThe present work deals with an experimental investigation on methane hydrate and carbon
dioxide hydrate formation in presence of two different types of sand, which acted as seabed
simulators. The first typology of sand consists in pure quartz and is commonly used for
laboratory experiments on gas hydrate. The other type is named TS-2 and originated from
the Tunisian seabed of the Mediterranean Sea. It is silica-based (≈99%), however it also
contains other elements and its grains have different shape, size and porosity. Experiments
were carried out in order to verify if the specific characteristic of the seabed, in which the
hydrate reservoir is present, may intervene or not on the CO2/CH4 replacement process and if
such contribution may improve or reduce the process efficiency. Results proved that physical
and chemical properties of materials which composed the seabed, may strongly intervene
on the replacement process. In particular, experiments revealed that sand TS-2 acted as
kinetic and thermodynamic inhibitor for methane hydrate formation, while it promoted
CO2 hydrate formation under the kinetic point of view. In this sense, sand TS-2 represents a
strong ally for improving the replacement efficiency, due to its capability to both improving
the kinetic of the process and reducing the methane hydrate re-formation phenomena. The
present study revealed that, with a simple analysis of properties of sediments containing
hydrate reservoirs, it would be possible to establish the convenience of intervening with a
replacement strategy instead of a classical application for simple methane recover
HOW THE LOWERING OF PRESSURE OR THE FORMATION OF ICE AFFECT THE FORMATION AND DISSOCIATION OF METHANE HYDRATES: VARIATIONS IN TERMS OF METASTABLE PHASE AND ANOMALOUS SELF-PRESERVATION
No full textThis research proposes the multiple and consecutive formation of methane hydrates with the same gas–water
mixture and in a small-scale reactor. The scope consisted of better exploring how the variation of the initial pressure
of the system modified the formation and dissociation processes. One of the most promising solutions for gas
hydrates exploitation consists in the replacement of methane with carbon dioxide. This process inevitably requires
the formation of CO2
hydrates; thus, more details and analyses on the driving forces of the formation process are
required in order to improve the overall replacement efficiency. Also, the competition with ice was analyzed. Nine
tests were carried out in order to consider all these parameters. Each test was characterized thermodynamically
and the time required to complete each specific phase of the process was measured. The tests were carried out at
two different initial pressures: 60 and 50 bar, respectively. It was found that the main effect, associated with a
lower initial pressure and/or with the presence of ice, is the stretching of the so-called metastable phase; since the
local conditions became suitable for the formation of hydrates, the system required more time to begin in these
latter cases. In the tests made at 60 bar, the delay in hydrate formation related to the metastable phase was equal
to 0.73–1.04 h, while it ranged from 7.29 to 15.16 h in the tests made at 50 bar. Moreover, the results proved that
the presence of ice hindered the formation process by reducing the heat transfer rate and by lowering the volume
available for the process. Conversely, the begin of the dissociation phase was similar in all the experiments: the
anomalous self-preservation, or the capability of the system to preserve itself even outside the hydrate stability
zone was not observed, proving again that these limiting factors (lower initial pressure and ice formation) mainly affected the process during the formation phase
Formation rate as parameter to distinguish nucleation from hydrate massive growth phase: Experimental investigation in presence of two different porous media
No full textMethane hydrate formation was tested in presence of two different porous media. Despite silica, the second sand
contains several other compounds and grains are different from each other. The evaluation of the hydrate formation rate along the whole experiment, allowed to well distinguish the nucleation from the hydrate growth
phase, without further analyses: all diagrams presented an almost equal trend. As soon the reaction begun,
elevated formation rate values were measured. In almost all tests, after a few tens of minutes, values dropped and
then assumed a horizontal trend, thus defining a “elbow point”, which was considered the key element to
separate the two different phases. Analyses about moles of hydrate formed, together with pressure and temperature trends over time, revealed that this natural sand was a weak thermodynamic inhibitor. Conversely, the
hydrate formation rate proved that it acted as strong kinetic inhibitor, in particular during the hydrate growth
phase
The use of sodium chloride as strategy for improving CO2/CH4 replacement in natural gas hydrates promoted with depressurization methods
No full textNatural gas hydrates represent a valid opportunity in terms of energy supplying, carbon dioxide permanent storage and climate change contrast. Research is more and more involved in performing CO2 replacement competitive strategies. In this context, the inhibitor effect of sodium chloride on hydrate formation and stability needs to be investigated in depth. The present work analyses how NaCl intervenes on CO2 hydrate formation, comparing results with the same typology of tests carried out with methane, in order to highlight the influence that salt produced on hydrate equilibrium conditions and possibilities which arise from here for improving the replacement process efficiency. Sodium chloride influence was then tested on five CO2/CH4 replacement tests, carried out via depressurization. In relation with the same typology of tests, realised in pure demineralised water and available elsewhere in literature, three main differences were found. Before the replacement phase, CH4 hydrate formation was particularly contained; moles of methane involved were in the range 0.059–0.103 mol. On the contrary, carbon dioxide moles entrapped into water cages were 0.085–0.206 mol or a significantly higher quantity. That may be justified by the greater presence of space and free water due to the lower CH4 hydrate formation, which led to a more massive new hydrate structure formation. Moreover, only a small part of methane moles remained entrapped into hydrates after the replacement phase (in the range of 0.023–0.042 mol), proving that, in presence of sodium chloride, CO2/CH4 exchange interested the greater part of hydrates. Thus, the possibility to conclude that sodium chloride presence during the CO2 replacement process provided positive and encouraging results in terms of methane recovery, carbon dioxide permanent storage and, consequently, replacement process efficiency
Os clowns de Fellini: a porção palhaça da subjetividade
No full textTese (doutorado) - Universidade Federal de Santa Catarina, Centro de Comunicação e Expressão, Programa de Pós-graduação em Literatura, Florianópolis, 2013Trata-se uma leitura dos filmes de Federico Fellini, fundamentada na teoria da imagem cinematográfica de Gilles Deleuze e na relação entre magia e felicidade em Giorgio Agamben. É nas imagens-sonho e nas imagens-lembrança que trazem para a tela os clowns de Fellini que busco os germes de um cristal em formação que nos oferece a vida enquanto espetáculo, mas também, paradoxalmente, em toda sua espontaneidade. Por ser a imagem-cristal uma operação fundamental do tempo, no sentido em que nela se pode ver a coexistência entre o presente e o passado, é no carnaval e nas festas de loucos, na história do riso, da máscara e do circo que busco elementos para compreender a importância dos clowns no contexto dos filmes de Fellini, incluindo seus escritos e suas declarações. Depois, acompanho a suposta desaparição da graça e do inumano na virada epistemológica que marca os anos finais da era clássica: é aí que Bakhtin, em seu estudo sobre Rabelais, encontra uma mudança radical de atitude em relação ao riso, que já não pode mais fazer parte do sério; aí também Foucault analisa a exclusão da loucura, no cogito cartesiano. Não por acaso, é também aí que Rossi e Yates inserem a passagem da magia à ciência. Nesse contexto faço a leitura dos clowns de Fellini buscando enxergar neles "o passado que se conserva e assume todas as virtudes do começo e do recomeço". Esse passado que essas figuras conservam pode nos fazer contemporâneos da criança que fomos e nos fazer ver que "o que podemos alcançar por nossos méritos e esforços não pode nos tornar realmente felizes".<br
Cohomology of D-complex manifolds
No full textIn order to look for a well-behaved counterpart to Dolbeault cohomology in D-complex geometry, we study the de Rham cohomology of an almost D-complex manifold and its subgroups made up of the classes admitting invariant, respectively anti-invariant, representatives with respect to the almost D-complex structure, miming the theory introduced by Li and Zhang (2009) in [20] for almost complex manifolds. In particular, we prove that, on a 4-dimensional D-complex nilmanifold, such subgroups provide a decomposition at the level of the real second de Rham cohomology group. Moreover, we study deformations of D-complex structures, showing in particular that admitting D-Kähler structures is not a stable property under small deformations
Effects of injecting gaseous CO2 on natural gas hydrate reservoirs: comparison of differences in clathrate dissociation behaviour.
No full textThe present work deals with the investigation of effects produced by carbon dioxide hydrate presence around a natural gas hydrate deposit. A laboratory scale reactor was performed to reproduce conditions feasible for hydrate formation. Six experimental tests were made. Firstly methane hydrate were formed and, then, temperature was increased to describe its relation with hydrate dissociation. In the second group of tests, after methane hydrate formation, carbon dioxide was injected, to form a hydrate shell around the present methane nucleus; then temperature was increased for generating hydrate dissociation. Finally a complete CO2 replacement process was carried out in the third group of tests. In case of only methane hydrate presence, the temperature increase caused an abundant pressure increase (due to water cages dissociation). The same temperature increase did not provoke any dissociation in presence of CO2 hydrate. In the third group of tests, the gas-chromatographic analysis of gas mixture, present inside hydrate after the CO2 replacement process completion, revealed a consistent presence of methane. Results clearly show how the presence of CO2 hydrate hinder the methane hydrate dissociation, even if the thermodynamic conditions are not suitable for their stability
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